The derailleur system currently used in bicycles has been adequately engineered to allow the chain to smoothly move to an adjacent sprocket at selected locations around each sprocket. Each sprocket is modified to provide clearance for chain shifting with a small plurality of reduced size chain pickup teeth with modified shape at selected locations on the sprocket, but with adjacent normally sized teeth providing adequate chain grip and strength around the rest of the sprocket's outer periphery or circumference.
The chain travel is directed by a movement of the derailleur guiding the chain from the sprocket it is on and feeding the chain onto an adjacent larger sprocket when decreasing the speed or a smaller sprocket when increasing the speed.
The bicycle derailleur devices are physically mounted on the frame of the bicycle and its movement is generally operated by a guide wire or cable extending from a shifting lever attached on the handle bar to the derailleur. When the rider selects a new sprocket, he moves the lever moving the wire or cable which in turn repositions the derailleur to effect a sprocket change. The derailleur is a sophisticated device that has a fixed structure attached to the frame or rear axle and a movable structure that is held in position by at least one double pivot with multiple heavy duty springs that allow the movable elements to rotate or move in the path of a parallelogram created by two opposing plates hinged to move parallel relative to the other. This movement enables the chain guide portion of the derailleur to move relative to the sprocket nest in a relatively uniform path in relation to the guide and the sprockets while minimizing twisting the chain.
In order to function properly, the chain slack that exists when the chain is on the sprocket must be taken up. The amount of chain slack is reduced as the chain moves to larger sprockets. In bicycles, the derailleur chain guide system is positioned to extend well below the axle and frame. This enables the chain to serpentine through the low hanging derailleur guide system by having the chain extend well below the largest of the sprockets.
These prior art derailleur devices allow the chain to be moved along the various sized sprockets with relative ease. In U.S. Pat. No. 7,780,558 entitled “Bicycle Rear Derailleur” assigned to Shimano Inc.; a rather detailed description is provided of these rather sophisticated devices for changing sprockets on a bicycle. Shimano is a world leader in the manufacture of derailleurs and is known for providing some of the best devices for changing sprockets on chain driven bicycles. Their rear derailleurs are engineering marvels exhibiting good reliability and durability.
These excellent products, however, because they are so well made with much sophistication are costly. A need exists to provide a simpler, less complex derailleur design that is reliable and durable.
Recently, a new generation of scooters and bicycles has been developed with reciprocating foot pedals. These vehicles, particularly the scooters, have frames that have a very low center of gravity to make the vehicles more stable. The frames are so low to the ground that the prior art derailleurs conventionally positioned below the rear sprocket nest are virtually useless. Nevertheless, these vehicles need a shifting device to allow the rider to selectively change speeds.
It is therefore an objective of the present invention to provide a derailleur type device and chain slack take-up assembly that does not extend into the ground clearance of the vehicle. It was a further objective of the present invention to provide a reliable and less complex device that can be manufactured at an affordable cost.
It was still another objective of the present invention to provide a derailleur device with a large range of motion to facilitate the range of rear sprocket sizes in a single rear sprocket nest equivalent to present bicycle derailleurs.
Another objective of the invention is to have the chain feed to the derailleur chain take-up directed so as to be fed close to the centerline of the drive sprocket.
These and other features of the derailleur system of the present invention are described as follows.